Recently, in high-frequency applications using a microwave band or a milliwave band as a carrier, for example, in wireless LAN or various communication terminals, reduction in size and thickness of equipment and circuit board has been demanded. In a circuit board for such high-frequency applications, filter elements such as a low-pass filter (LPF), a high-pass filter (HPF) and a band-pass filter (BPF) are designed with a distributed constant, for example, using a microstrip line or a strip line that enables relatively high space-saving, instead of using a lumped constant design using chip components like an inductor and a capacitor.
For example, a circuit board 100 shown in FIG. 1 has a BPF 101 of a flat structure, as a filter element designed with a distributed constant. In this circuit board 100, conductor patterns 103 made of copper or nickel plated with gold are formed as microstrip lines on a dielectric board 102 such as a printed board or a ceramic board, thus constituting the BPF 101. On the entire back side of the dielectric board 102, a ground part (not shown) is formed.
With such a BPF 101, it is possible to selectively transmit a signal of a desired frequency band by optimizing the shape of the conductor patterns 103. Since this BPF 101 is a part of the whole pattern wiring formed on the dielectric board 102 and has a flat structure, the BPF 101 can be collectively formed when forming the pattern wiring on the dielectric board 102, for example, by print processing, lithography processing or the like.
In the circuit board 100 shown in FIG. 1, since the BPF 101 has a flat structure and the conductor patterns 103 are arrayed with an overlap of substantially ¼ of a passing wavelength λ, the length of the conductor patterns 103 is prescribed by the passing wavelength λ. In the circuit board 100, the conductor patterns 103 need to have a certain length and it is difficult to reduce the occupied area of the conductor patterns 103. Therefore, area-saving is limited.
Thus, in a circuit board 100 shown in FIGS. 2A to 2D, it is proposed to save the area by using a BPF 111 as a filter element that requires a smaller occupied area. This BPF 111 has a so-called tri-plate structure, which is a three-layer structure in which resonator conductor patterns 114 arranged substantially parallel to each other are formed in an inner layer of a multilayer board 113 such as a multilayer printed board.
Specifically, in the BPF 111, a pair of resonator lines 114 has an impedance step structure in which low-impedance lines (thick lines) 115 and high-impedance lines (thin lines) 116 are connected with each other near a substantially central part in the longitudinal direction, and feeder wirings 117 are connected to parts near substantially central parts of the high-impedance lines 116, respectively, as shown in FIG. 2C. In this BPF 111, the pair of resonator lines 114 is held between two ground parts 118a, 118b as ground conductors from above and below, with insulating layers 112 provided between the resonator lines 114 and the ground parts 118a, 118b. In this BPF 111, the two ground parts 118a, 118b are connected with each other in the form of interlayer connection by via-holes 119 surrounding the pair of resonator lines 114, and the resonator lines 114 in the layer are shielded by the ground parts 118a, 118b and the via-holes 119.
In this filter circuit 110, the pair of resonator lines 114 is constructed as lines having a length substantially ¼ of the passing wavelength λ are arranged in parallel and then capacitive-coupled. As the pair of resonator lines 114 has the impedance step structure, the length of the lines arranged in parallel can be made equal to or less than the passing wavelength λ, and the occupied area of the BPF 111 can be reduced to realize miniaturization.
In this filter circuit 110, when the BPF 111 is shown in the form of an equivalent circuit, parallel resonance circuits are capacitive-coupled, as shown in FIG. 3. Specifically, a parallel resonance circuit PR1 including a capacitor C1 and an inductance I1 connected between one of the two resonator lines 114 and the ground parts 118a, 118b, and a parallel resonance circuit PR2 including a capacitor C2 and an inductance I2 connected between the other of the two resonator lines 114 and the ground parts 118a, 118b, are capacitive-coupled via a capacitor C3 generated between the pair of resonator lines 114.
In the above-described filter circuit 110, as the impedance ratio between the low-impedance lines 115 and the high-impedance lines 116 of the pair of resonator lines 114 is increased, the lines arranged in parallel can be reduced. Therefore, the occupied area of the filter element can be reduced to realize miniaturization. Specifically, by forming the high-impedance lines 116 that are much thinner than the low-impedance lines 115, it is possible to further miniaturize the filter circuit 110.
In the filter circuit 110, since metal layers of the above-described resonator lines 114 formed by a thick film forming technique such as a plating method are patterned by etching processing or the like, it is difficult to reduce the thickness of the high-impedance lines 116 to 0.075 mm or less, thus limiting the miniaturization.
In this filter circuit 110, when the thickness of the high-impedance lines 116 of the pair of resonator lines 114 is reduced to the minimum possible level, it is difficult to form the high-impedance lines 116 with high accuracy, and reduction in yield and deterioration in filter characteristic may occur.